The compressor section of a gas turbine engine inducts air and compresses it through alternating rows of stationary vanes and rows of rotating blades. Each row of blades is formed by a plurality of blades attached in a circumferential array to a rotor disc.
Referring to FIG. 1, each compressor blade 10 is comprised of an airfoil 12 and a root 14. The root 14 can be configured to be slidably received into a respective axial groove 16 in a compressor disc 18. The root 14 and the groove 16 can be configured for interlocking engagement. As is shown in FIG. 1, at least a portion of the root 14 is configured as a male dovetail, and at least a portion of the groove 16 is configured as a corresponding female dovetail. As a result of such an arrangement, movement of the blade 10 is restrained by the groove 16 of the disc 18 in the radial and circumferential directions relative to the axis of rotation of the disc. However, restraint of the blade 10 in the axial direction, sometimes referred to as locking, requires a separate device.
Locking in the axial direction has been achieved by using a blade closing key 20 in connection with one or more of the blades in the row. In some instances, a closing key may be used only in connection with the last blade installed in the row. To accommodate the blade closing key 20, a notch 22 is provided in an upper portion of each blade root 14 and a circumferential slot 24 is provided about a portion of the rotor disc 18. When the blade 10 is inserted in the disc 18, the slot 24 and the notch 22 are aligned and collectively define a cavity 26 to receive the closing key 20. The cavity 26 is closed at one circumferential end by the notch 22 in the blade root 14. The cavity 26 is closed at its opposite circumferential end by a portion of the root of a neighboring blade (not shown). The closing key 20 is received in the cavity 26.
Referring to FIG. 2, one known closing key 20′ is a two piece construction that has a first key piece 28 (also called a tail piece) and a second key piece 30 (also referred to a head piece). The first key piece 28 has a base body 32 with a first straight finger 34 protruding from a first side surface 36 thereof. Likewise, the second key piece 30 has a base body 38 with a second straight finger 40 protruding from a second side surface 42 thereof. The second key piece 30 includes a tab 44 adapted to be received in the notch 22 in the blade root 14. Each of the base bodies 32, 38 have a cross-sectional shape adapted for interlocking engagement with the cavity 26. For example, the base bodies 32, 38 have been configured as male dovetails for interlocking engagement with the cavity 26.
To install the closing key 20′ in the cavity 26, the first key piece 28 is initially inserted in the cavity 26 and then the second key piece 30. Once in the cavity 26, the first and second key pieces 28, 30 are collapsed, that is, the first and second key pieces 28, 30 are brought together so that the end of the first finger 34 substantially abuts the second side surface 42 of the base body 38 of the second piece 30 and so that the end of the second finger 40 substantially abuts the first side surface 36 of the base body 32 of the first piece 28. In such case, the first and second fingers 34, 40 will be generally parallel to each other along their lengths. Further, in order to lock the blade 10, the first and second key pieces 28, 30 are moved away from each other such that the tab 44 of the second key piece 30 is moved in the notch 22 of the blade root 14 to lock the blade 10. The ends of the fingers 34, 40 are brought into operative engagement with each other. In doing so, the fingers 34, 40 bend, as is shown in FIG. 3. This expanded length of the closing key 20′ is held fixed by the opposing bending forces exerted by each of the pieces 28, 30.
However, in certain circumstances, the first and second key pieces 28, 30 may be able to rotate within the cavity 26. Such rotation, which is sometimes referred to as window cocking, may arise for any of a number of reasons. For instance, rotation of the first and second key pieces 28, 30 can be due to machining tolerances in forming the first and second key pieces 28, 30 and/or the cavity 26. Alternatively or in addition, rotation of the first and second key pieces 28, 30 can arise if there is a decrease in the width of the first and second key pieces 28, 30 and/or if the width of the cavity 26 increases. Such changes in width can be due to wear and/or corrosion.
If one or both of the key pieces 28, 30 rotates a sufficient amount, then the fingers 34, 40 may no longer oppose one another and become disengaged so that the fingers 34, 40 slide past each other. In such case, the opportunity is provided for the first and second key pieces to move toward each other, which shortens their combined length, similar to when the pieces 28, 30 are collapsed to facilitate installation. When this happens, it is possible for the tab 44 to move out of the blade notch 22, allowing the blade 10 to become unlocked, that is, the blade 10 is permitted to move axially within the groove 16 in the compressor disc 18. If the blade 10 becomes liberated, it can cause significant damage to other components in the compressor and can force engine shutdown.
Thus, there is a need for a closing key configured to minimize such concerns.